1. Field of the Invention
The present invention relates generally to integrated circuits and multilayer structures for fabricating integrated circuits. More particularly, the invention relates to particular multilayer structures and methods of fabricating multilayer structures to suppress channel field inversion of isolation implants between transistors in integrated circuits (ICs).
2. State of the Art
Typically, integrated circuits comprise a large number of devices (e.g., transistors) fabricated on a silicon substrate that are electrically isolated from one another using, for example, a field oxide layer. Electrical isolation of the transistors is typically achieved by first doping a silicon substrate channel between transistors to impede carriers from moving through the channel. Thus if the transistors are n-type, the isolation channel is doped with a p-type dopant such as boron. Oxide layers are also generated between transistors to insulate the doped isolation channel from voltages applied during fabrication, packaging or operation of the integrated circuit.
Typically, transistors are selectively connected by photolithographically patterning metallization layers as metal traces. To permit routing of a metal trace over a multilevel substrate, two or more metallization levels are typically used to form each metal trace. Each metallization level is separated by a dielectric layer to isolate each metallization level and to prevent shorting of traces. Vias may be formed in the dielectric layer to complete traces in different metallization levels. These dielectric layers tend to conform to the surface of the traces thus forming valleys between the traces.
To planarize intermediate layers of the IC during fabrication, the dielectric layer is formed as an intermetal oxide sandwich interposed between each metallization layer. The intermetal oxide sandwich includes a spin-on-glass (SOG) layer which is applied as a liquid to fill in the valleys created by each metallization layer. An outer passivation layer is formed which protects the integrated circuit from ambient conditions such as moisture. This passivation layer may be comprised of a silicon nitride, for example.
The foregoing integrated circuit is susceptible to current leakage between adjacent transistors separated by isolation areas. A threshold voltage is associated with the underlying substrate and the thickness of the oxide layer. When a voltage is applied to the isolation oxide such that its threshold voltage is exceeded, inversion of the channel substrate beneath the oxide layer occurs and carriers travel between adjacent transistors. This effect impairs integrated circuit performance, lowers manufacturing yield and raises costs.
There are many different circumstances wherein a voltage is applied to the oxide layer which causes isolation breakdown. For example, a reaction between the nitride passivation layer and the SOG layer creates a charge that leads to inversion of the isolation channel. During the manufacturing of the IC packages, certain high temperature operations cause hydrogen to be released from the nitride passivation layer. This free hydrogen diffuses downward and reacts with carbon in the organic SOG layer. This reaction results in the formation of a positive charge which is sufficient to cause isolation breakdown. Additionally, there are many fabrication operations wherein charge can be induced in the isolation oxide such that the underlying channel is inverted.
In co-pending commonly assigned U.S. patent application Ser. No. 07/476,089, U.S. Pat. No. 5,057,897 filed on Mar. 5, 1990, exemplary solutions to the foregoing isolation breakdown problem are proposed. For example, the intermetal oxide sandwich is modified so that one or both of the oxide layers which sandwich the SOG layer are enriched with silicon. Dangling bonds of the silicon neutralize charge created by the reaction of the hydrogen and carbon, particularly if the enriched layer is below the SOG layer. The silicon-enriched, intermetal oxide layer is created by, among other methods, adding additional silicon-bearing reagent during plasma deposition.
Although the introduction of silicon enriched oxide reduces isolation breakdown due to the reaction between the nitride passivation layer and the SOG layer, it would be desirable to further suppress isolation breakdown caused by other phenomena without altering properties (e.g., step coverage, stress, and etch rates) of the intermetal oxide layers. Also, it would be desirable to simplify manufacturing of the integrated circuit, reduce manufacturing costs, and shield the field oxide layer from voltages or charges introduced into layers other than the SOG layer.